Implantable Cardioverter Defibrillators (ICDs) are implanted in patients susceptible to cardiac tachyarrhythmias including atrial and ventricular tachycardias and atrial and ventricular fibrillation. Such devices typically provide cardioversion or defibrillation by delivering low voltage pacing pulses or high voltage shocks to the patient's heart, typically about 500–800V. The ICD operates by detecting a fast heart rate or tachyarrhythmia, upon which a battery within the device housing is coupled via an inverter to a high voltage capacitor or capacitor pair to charge the capacitors. When the capacitor reaches a desired voltage, charging is stopped and the capacitors are discharged under control of a microprocessor to provide a therapeutic shock to the patient's heart. The capacitance of the capacitor is established to deliver a shock with an energy of 30–40 joules. In conventional devices, smaller capacitors are inadequate to deliver sufficient charge to provide effective therapy.
The time between detection of a cardiac and the delivery of the shock is an important concern. It is believed that more prompt therapy provides more effective results. For instance, a patient who experiences tachyarrhythmia may lose consciousness before therapy is delivered. This can lead to injuries from falling, or vehicle accidents should the patient be driving. In addition, it is believed that a major cause of death among some implanted patients is the progression to electromechanical disassociation (EMD), in which therapy restores electrical activity without restoring hemodynamic function, so that the affected cardiac cells fail to regain their blood-pumping function.
To provide shorter charge times and thus faster therapy delivery, conventional technology has employed larger batteries that deliver current to the capacitor at a higher rate. This has the disadvantage of requiring a larger overall implant package, or at least limits the amount of miniaturization that would otherwise be desired for patient comfort. Another approach to reduce charge time intervals has been to employ more sophisticated battery technology that provides higher current rates for a given size. However, this suffers from higher cost, and also limited miniaturization. Alternative approaches have employed networks of batteries having different characteristics that allow more rapid charging while not significantly limiting battery life. However, these approaches contribute to device complexity and cost.
Medical researchers (Gradaus, et al., PACE January 2002) have discovered that the defibrillation threshold (DFT) in humans is not constant, but is lower as the time interval to therapy is reduced. DFT is the amount of energy (typically measured in joules) required for effective therapy. The observed DFT reduction starts occurring only below very brief time intervals (less than about 8 seconds.) Thus, existing technology lacks the capability to charge capacitors in such a brief interval without accruing some of the disadvantages noted above regarding large and or expensive batteries. Therefore, there remains a need for a fact acting charging system that does not sacrifice size, cost, or complexity.